A number of oxides of nitrogen are known which are relatively stable at ambient conditions and two of these, nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are principal contributors to atmospheric pollution. In this specification, they are referred to generically as NO.sub.x for convenience. They are usually formed in internal combustion engines as well as in furnaces, boilers and incinerators in which high temperature combustion processes occur. Although the concentrations of NO.sub.x is the exhaust gases of these combustion processes are relatively low, the aggregate amounts discharged into the atmosphere are sufficient to cause significant pollution problems. Reduction of NO.sub.x emissions is therefor a major objective in the control of industrial and automotive pollution.
Various reducing agents such as ammonia, carbon monoxide, hydrogen and hydrocarbons may be used to convert NO.sub.x to molecular nitrogen (N.sub.2) in the presence of suitable catalysts. A number of metals and metal oxides are known to be catalytically active for the reaction, including platinum, rhodium, vanadium oxide, tungsten and titanium oxide. See Bosch, H. and Janssen, F., "Catalytic Reduction of Nitrogen oxides-A Review of the Fundamentals and Technology", Catalysis Today, Vol. 2, No. 4 (Mar. 1988), pp. 369-531. The metal component is conventionally supported on a porous carrier such as silica, alumina, zirconia or a zeolite such as ZSM-5. See Groeneveld, M. J. et al., "Preparation, Characterization and Testing of New V/Ti/SiO2 Catalysts for Denoxing and Evaluation of Shell Catalyst S-995, Catalysis: Theory to Practice, Proceedings of the 9th International Congress on Catalysis, Vol. 4, pp. 1743-1749 (1988). A number of NO.sub.x reduction processes are referred to in U.S. Pat. No. 4,929,586 (Hegedus). U.S. Pat. No. 4,778,665 (Krishnamurthy) discloses a selective catalytic reduction process using an intermediate poor size zeolite with a metal component such as platinum as the catalyst.